I'm rather startled that they never called on the Lord.

THey stated in the movie that this entity was physical in nature, just from another dimension... So i dont think your God could help with that as he tends to not deal with the physical.

Habataitara modorenai to itte
Mezashi-ta no wa aoi aoi ano sora

I don't know if I'd say it's "likely" that God exists. But maybe it would make the possibility *more* likely.

I used to be a Christian, but now I'm an Agnostic Atheist. I believed in God, Jesus, angels, demons, and other such beings, but I no longer believe in anything spiritual or supernatural. But, if something which was undoubtedly supernatural happened to me, like this demonic apparition entering my house, it would certainly get me thinking about the supernatural realm, all the other things I've dismissed, maybe even God, and might even get me back in a church. But it's tough to say how I'd react in such a situation.

But to your point, are you talking specifically about the God of the Bible? If you were to find out that demons do exist, why would it make it more likely he exists, instead of Allah, or Vishnu, or Zeus? I wouldn't say it suddenly makes any one spiritual belief system more likely. Just that it certainly opens the door to a lot of other possibilities. But I will admit, the God of the Bible is probably the one I would start reconsidering first, simply because I was raised Christian.